Gets the value from the previous operation.

import Batch = require("nativescript-batch");

var b = Batch.newBatch(function(ctx) {
    // ctx.prevValue = undefined
    ctx.nextValue = "TM";
}).then(function(ctx) {
    // ctx.prevValue = "TM"
    
    // something that does NOT set
    // ctx.nextValue property
}).then(function(ctx) {
    // ctx.prevValue = undefined
    ctx.nextValue = "MK";
}).then(function(ctx) {
    // ctx.prevValue = "MK"
    ctx.nextValue = "YS";
}).then(function(ctx) {
    // ctx.prevValue = "YS"
});

b.start();